通过字符串生成 EF orderby 表达式

Question

我想通过字符串参数生成表达式，一些代码如：

private Expression<Func<Task, T>> Generate(string orderby)
{
    switch (orderby)
    {
        case "Time":  
            return t => t.Time;
        case "Money":
            return t => t.RewardMoney;
        default:
            return t => t.Id;
    }
}

然后调用它：

_context.Items.OrderBy(Generate("Money"));

但它无法编译！我把 T 改成 object。

private Expression<Func<Task, object>> Generate(string orderby)

然后它可以编译，但它不起作用。

System.NotSupportedException：无法将类型“System.Int32”转换为类型“System.Object”。 LINQ to Entities 仅支持转换 EDM 基元或枚举类型。

Answer 1

使用reflection和expression-trees可以提供参数然后调用OrderBy函数，而不是返回Expression<Func<Task, T>>然后调用OrderBy。

请注意，OrderBy 是一种扩展方法，并已在 System.Linq.Enumarable 和 System.Linq.Queryable 类中实现。第一个用于linq-to-objects，后者用于linq-to-entities。 entity-framework 需要查询的表达式树才能将其转换为 SQL 命令。所以我们使用Queryable 实现。

可以通过扩展方法来完成（添加为cmets的解释）：

public static IOrderedQueryable<TSource> OrderBy<TSource>(
       this IQueryable<TSource> query, string propertyName)
{
    var entityType = typeof(TSource);

    //Create x=>x.PropName
    var propertyInfo = entityType.GetProperty(propertyName);
    ParameterExpression arg = Expression.Parameter(entityType, "x");
    MemberExpression property = Expression.Property(arg, propertyName);
    var selector = Expression.Lambda(property, new ParameterExpression[] { arg });

    //Get System.Linq.Queryable.OrderBy() method.
    var enumarableType = typeof(System.Linq.Queryable);
    var method = enumarableType.GetMethods()
         .Where(m => m.Name == "OrderBy" && m.IsGenericMethodDefinition)
         .Where(m =>
         {
            var parameters = m.GetParameters().ToList();
            //Put more restriction here to ensure selecting the right overload                
            return parameters.Count == 2;//overload that has 2 parameters
         }).Single();
    //The linq's OrderBy<TSource, TKey> has two generic types, which provided here
    MethodInfo genericMethod = method
         .MakeGenericMethod(entityType, propertyInfo.PropertyType);

    /*Call query.OrderBy(selector), with query and selector: x=> x.PropName
      Note that we pass the selector as Expression to the method and we don't compile it.
      By doing so EF can extract "order by" columns and generate SQL for it.*/
    var newQuery = (IOrderedQueryable<TSource>)genericMethod
         .Invoke(genericMethod, new object[] { query, selector });
    return newQuery;
}

现在您可以像其他任何重载一样调用 OrderBy 的重载。
例如：

var cheapestItems = _context.Items.OrderBy("Money").Take(10).ToList();

翻译为：

SELECT TOP (10)  {coulmn names} FROM  [dbo].[Items] AS [Extent1] 
       ORDER BY [Extent1].[Money] ASC

此方法可用于定义 OrderBy 和 OrderByDescending 方法的所有重载以具有 string 属性选择器。

Answer 2

您可以尝试将Generate 方法转换为泛型方法：

private Expression<Func<Task, TResult>> Generate<TResult>(string orderby)
{
     switch (orderby)
     {
        case "Time":  
          return t => t.Time;
        case "Money":
          return t => t.RewardMoney;
        default:
         return t => t.Id;
     }
}

所以，如果你调用这个方法，你需要指定你想要排序的属性的类型：

_context.Items.OrderBy(Generate<decimal>("Money"));

现在请记住，TResult 只能是基本类型或枚举类型。

Answer 3

我参考了 CodePlex 中旧的System.Linq.Dynamic codebase，从实现和调用的角度创建了一个相当简单的版本。当然是IQueryable<T>上的扩展方法

/*
using System;
using System.Linq;
using System.Linq.Expressions;
*/

public static IQueryable<T> OrderBy<T>(this IQueryable<T> query, string orderByExpression)
{
    if (string.IsNullOrEmpty(orderByExpression))
        return query;

    string propertyName, orderByMethod;
    string[] strs = orderByExpression.Split(' ');
    propertyName = strs[0];

    if (strs.Length == 1)
        orderByMethod = "OrderBy";
    else
        orderByMethod = strs[1].Equals("DESC", StringComparison.OrdinalIgnoreCase) ? "OrderByDescending" : "OrderBy";

    ParameterExpression pe = Expression.Parameter(query.ElementType);
    MemberExpression me = Expression.Property(pe, propertyName);

    MethodCallExpression orderByCall = Expression.Call(typeof(Queryable), orderByMethod, new Type[] { query.ElementType, me.Type }, query.Expression
        , Expression.Quote(Expression.Lambda(me, pe)));

    return query.Provider.CreateQuery(orderByCall) as IQueryable<T>;
}

这里是如何使用它的示例，已针对 Entity Framework Core 3 进行了测试：

IQueryable<Person> query = dbContext.People;
query = query.OrderBy("FirstName"); // ORDER BY FirstName

IQueryable<Person> query = dbContext.People;
query = query.OrderBy("FirstName ASC"); // ORDER BY FirstName

IQueryable<Person> query = dbContext.People;
query = query.OrderBy("FirstName DESC"); // ORDER BY FirstName DESC

Answer 4

使用通用方法。由于 lambda 表达式只能分配给强类型的委托或表达式，因此我们必须使用相应的 temp。然后我们可以将此 temp 分配给类型为 object 的变量。最后我们可以通过强制转换为结果类型来返回结果。

public Expression<Func<Task, TResult>> Generate<TResult>(string orderby)
{
    object result;
    switch (orderby) {
        case "Time":
            Expression<Func<Task, DateTime>> temp1 = t => t.Time;
            result = temp1;
            break;
        case "Money":
            Expression<Func<Task, decimal>> temp2 = t => t.RewardMoney;
            result = temp2;
            break;
        default:
            Expression<Func<Task, int>> temp3 = t => t.Id;
            result = temp3;
            break;
    }
    return (Expression<Func<Task, TResult>>)result;
}

Answer 5

public static IQueryable<T> OrderByHelper<T>(this IQueryable<T> source, string propertyName, string sortDirection)
    {

        try
        {
            if (source == null)
            {
                return source;
            }
            if (propertyName == null)
            {
                return source;
            }

            propertyName = propertyName.First().ToString().ToUpper(new CultureInfo("en-US", false)) + propertyName.Substring(1);
            var type = typeof(T);
            var arg = Expression.Parameter(type, "x");

            var propertyInfo = type.GetProperty(propertyName);
            var mExpr = Expression.Property(arg, propertyInfo);
            type = propertyInfo.PropertyType;

            var delegateType = typeof(Func<,>).MakeGenericType(typeof(T), type);
            var lambda = Expression.Lambda(delegateType, mExpr, arg);

            var methodName = !string.IsNullOrEmpty(sortDirection) && sortDirection.ToLower(new CultureInfo("en-US", false)) == "desc" ? "OrderByDescending" : "OrderBy";
            var orderedSource = typeof(Queryable).GetMethods().Single(
                method => method.Name == methodName
                        && method.IsGenericMethodDefinition
                        && method.GetGenericArguments().Length == 2
                        && method.GetParameters().Length == 2)
                .MakeGenericMethod(typeof(T), type)
                .Invoke(null, new object[] { source, lambda });

            return (IQueryable<T>)orderedSource;
        }
        catch (Exception)
        {

            return source;
        }
    }